Remember how Mark Shuttleworth justified moving the window titlebar widgets to the left by claiming the space freed up on the right side could now be used for something else? On his blog, Shuttleworth unveiled what, exactly, Ubuntu's plans are: window indicators, or 'windicators'. In a nutshell, it comes down to having a tray area in every window.

There are just as many on linux (at least in gnome). The only mainstream OS I know of that doesn't have them out of the box is OSX, and even then, the first thing many osx users do is install growl, which is a framework to provide those notification popups.